Screening of wastewater is a common method of treatment at wastewater treatment plants. Wastewater screens have been used to remove large or inert solids for collection and disposal for over a century. Wastewater screening to remove such debris typically occurs prior to the activated sludge process. More recently, screening has also been conducted within the activated sludge process itself to select and remove inert material. U.S. Pat. Nos. 4,541,933, 5,824,222 and 7,569,147 each discuss variations of this concept for the removal of inert-materials from activated sludge. Another use of screens is described in U.S. Pat. No. 3,959,124, in which screened activated sludge solids can be returned to increase the solids retention time and the capacity of the overall process. In this process, the focus is on retention of activated sludge generally and not on selection of particular constituents in the activated sludge. None of these previously disclosed processes uses screens for both ‘selection’ and ‘retention’ of solids.
A current operational drawback of an activated sludge process is that the solids retention time is the same for all of the fractions in the activated sludge. This drawback has been overcome, for example, by using plastic media to retain organisms on biofilms for longer solids residence times. See, e.g., U.S. Pat. Nos. 5,458,779 and 7,854,843. Other options include the use of cyclones that can differentiate based on specific gravity. An improved selection process for retaining particular constituents of the activated sludge is desired.